Various proposals have hitherto been made for an apparatus for producing a corrugated tube of synthetic resin so that the outer circumferential surface of the tube has a corrugated cross-sectional profile. For example, there is known an apparatus comprising a roller assembly made up of plural rollers forming an imaginary cylinder, the rollers being each provided with plural convexed rings of the same size, and being rotated about their own axes at equal speed. The rollers are arranged parallel to one another so that the axes of the rollers are on the circumference of the imaginary cylinder. The axial directions of the rollers are circumferentially inclined and parallel to one another. A die is provided for continuously supplying a strip of synthetic resin in a molten state from the lateral side onto the periphery of the roller assembly. A driving motor continuously rotates the rollers at equal speed. There is also known an apparatus (Japanese Patent Publication No. 62-60261 (1987)) which further comprises a pressure roller profiled for meshing with the profile of the outer wall surface of each roller for fixing the shape of the strip of synthetic resin, means for cooling the synthetic resin as it is shaped to a tubular profile as it is moved along the outer periphery of the imaginary cylinder, and forming means for forming a smooth cylindrical-shaped synthetic resin on the inner side of the corrugated tube for providing a double-walled structure.
However, with the above-described known apparatuses, as the strip of synthetic resin is wound spirally and advanced along the axis of the roller assembly while being cooled and cured, the strip itself contracts in size due to cooling. Since the convexed rings formed on each roller are of the same size and arrayed at an equal interval on the roller, the synthetic resin strip is actually being cured under conditions in which the strip is pulled outwardly along the shape of each convexed ring. The result is that, as the strip of synthetic resin progresses axially along the roller assembly, the strip tends to be detached from the valley portions between the convexed rings to detract from molding stability, accuracy and durability of the ultimate corrugated tube.
Besides, with the above-described known apparatus, although each roller of the roller assembly is rotated at equal speed, a difference in circumferential speed of about 10 to 45% is produced between (a) circumferential speed at the apex of the convexed portion, and (b) speed in the recessed interstices between neighboring convexed portions of each roller, so that longitudinal creases (internal strain) are produced between the convexed portions and the recessed valley portions of the wall surfaces of the corrugated tube. As a result, the corrugated tube produced is lowered in durability, such as by reduced impact strength or pressure resistance.